1. Field of the Invention
The invention relates to a process for optimizing the frequency spectrum of pulses emitted by an emission device formed by a plurality of seismic implosion sources.
Seismic pulses generated by implosion sources are generally obtained by the sudden contraction in a liquid medium of a cavity in which there is a very low pressure with respect to the hydrostatic pressure.
2. Description of the Prior Art
This cavity may be defined by the walls of an immersed enclosure of variable volume of the type described for example in French Pat. No. 1 583 737.
It may also be obtained by injecting into the water mass a certain amount of super heated water vapor or by cavitation caused by the high speed discharge in the water of a certain volume of pressurized liquid.
As will be seen during the description and with reference to FIG. 1, the pulse generated by an implosion source has a first so called precursor peak corresponding, depending on the cases, to an overpressure or to an underpressure with respect to the hydrostatic pressure and which occurs on triggering off the source. The triggering moment is for example that when the mechanical means are actuated which held the enclosure in a maximum volume position, and when this latter begins to contract, or else that when water or super heated vapor is ejected at high speed. The peak is followed by a second pressure peak corresponding to the implosion properly speaking, and whose amplitude is much greater than that of the first peak. The amplitude ratio is generally of the order of ten. The interval of time T separating the times when the two peaks occur is generally designated a pseudo-period.
The pulse generated by an implosion source is brief and thus its frequency spectrum is wide, which is very favorable for carrying out seismic prospection operation at sea.
However, it can be observed that the frequency spectrum of the pulses due to implosions is limited in the low frequency range and has a hollow centered on a frequency f.sub.c whose value is related to the pseudo-period T by the relationship: EQU f.sub.c =(3/2T)
The value of the frequency f.sub.c is generally situated in the frequency band between 25 and 50 Hz.
The hollow in the frequency spectrum which is related to the existence of the precursor peak will be all the more pronounced the greater the ratio of the amplitude of this peak to the amplitude of the implosion peak. The existence of this hollow is very troublesome for using implosion for seismic prospection because the spectrum is reduced by a band of frequencies which are very useful for the quality of the recordings.
The process of the invention avoids the above mentioned disadvantages.